From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stan Hoeppner Subject: Re: Triple parity and beyond Date: Fri, 22 Nov 2013 02:38:19 -0600 Message-ID: <528F17FB.60909@hardwarefreak.com> References: <528A90B7.5010905@zytor.com> <528AA1EB.3010909@zytor.com> <528BCA2D.5010500@redhat.com> <73BEB41F-0FAC-4108-BEA9-DB6D921F6F55@cs.utk.edu> <528D61C5.70902@hardwarefreak.com> <528DCD36.1060808@hesbynett.no> Reply-To: stan@hardwarefreak.com Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <528DCD36.1060808@hesbynett.no> Sender: linux-raid-owner@vger.kernel.org To: David Brown , James Plank , Ric Wheeler Cc: Andrea Mazzoleni , "H. Peter Anvin" , linux-raid@vger.kernel.org, linux-btrfs@vger.kernel.org, David Smith List-Id: linux-raid.ids On 11/21/2013 3:07 AM, David Brown wrote: > For example, with 20 disks at 1 TB each, you can have: All correct, and these are maximum redundancies. Maximum: > raid5 = 19TB, 1 disk redundancy > raid6 = 18TB, 2 disk redundancy > raid6.3 = 17TB, 3 disk redundancy > raid6.4 = 16TB, 4 disk redundancy > raid6.5 = 15TB, 5 disk redundancy These are not fully correct, because only the minimums are stated. With any mirror based array one can lose half the disks as long as no two are in one mirror. The probability of a pair failing together is very low, and this probability decreases even further as the number of drives in the array increases. This is one of the many reasons RAID 10 has been so popular for so many years. Minimum: > raid10 = 10TB, 1 disk redundancy > raid15 = 8TB, 3 disk redundancy > raid16 = 6TB, 5 disk redundancy Maximum: RAID 10 = 10 disk redundancy RAID 15 = 11 disk redundancy RAID 16 = 12 disk redundancy Range: RAID 10 = 1-10 disk redundancy RAID 15 = 3-11 disk redundancy RAID 16 = 5-12 disk redundancy -- Stan